Demonstration of a neuronal projection from the entopeduncular nucleus to the substantia nigra of the rat

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

Vestibular responses in the macaque pedunculopontine nucleus and central mesencephalic reticular formation

The pedunculopontine nucleus (PPN) and central mesencephalic reticular formation (cMRF) both send projections and receive input from areas with known vestibular responses. Noting their connections with the basal ganglia, the locomotor disturbances that occur following lesions of the PPN or cMRF, and the encouraging results of PPN deep brain stimulation in Parkinson's disease patients, both the PPN and cMRF have been linked to motor control. In order to determine the existence of and characterize vestibular responses in the PPN and cMRF, we recorded single neurons from both structures during vertical and horizontal rotation, translation, and visual pursuit stimuli. The majority of PPN cells (72.5%) were vestibular-only (VO) cells that responded exclusively to rotation and translation stimuli but not visual pursuit. Visual pursuit responses were much more prevalent in the cMRF (57.1%) though close to half of cMRF cells were VO cells (41.1%). Directional preferences also differed between the PPN, which was preferentially modulated during nose-down pitch, and cMRF, which was preferentially modulated during ipsilateral yaw rotation. Finally, amplitude responses were similar between the PPN and cMRF during rotation and pursuit stimuli, but PPN responses to translation were of higher amplitude than cMRF responses. Taken together with their connections to the vestibular circuit, these results implicate the PPN and cMRF in the processing of vestibular stimuli and suggest important roles for both in responding to motion perturbations like falls and turns.

Effects of dopaminergic cell degeneration on electrophysiological characteristics and GAD65/GAD67 expression in the substantia nigra: different action on GABA cell subpopulations

The motor disturbances occurring in Parkinson's disease have been partially attributed to a hyperactivity of gamma-aminobutyric acid (GABA)-ergic nigral cells largely in the substantia nigra pars reticulata (SNr) secondary to the degeneration of dopaminergic nigrostriatal neurons. However, some aspects of this response remain unclear. In this work, different electrophysiological and neurochemical parameters were studied in GABAergic cells of the SN after unilateral nigrostriatal dopaminergic lesion using 6-hydroxydopamine injection in rats. Our data showed that 1) the SN under normal conditions contains different subsets of GABAergic cells according to their firing pattern and glutamic acid decarboxylase mRNA levels, and 2) the response of these GABAergic cell subgroups was different after the ipsi- and contralateral dopaminergic cell degeneration. These findings indicate a complex regulation of nigral GABAergic activity after nigrostriatal dopaminergic degeneration that probably involves local mechanisms, the nigro-striato-nigral loop, as well as interhemispheric mechanisms whose anatomical basis remains unstudied.

Response of GABAergic cells in the deep mesencephalic nucleus to dopaminergic cell degeneration: an electrophysiological and in situ hybridization study

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region located between the substantia nigra compacta and the superior colliculus. It contains GABAergic cells that share striatal afferents, thalamic and collicular efferents, as well as neurochemical and electrophysiological similarities, with those of the substantia nigra reticulata. In the present paper we used electrophysiological (firing rate and firing pattern) and morphological (densitometric analysis of in situ hybridization histochemical labeling for glutamic acid decarboxylase (GAD)65 and GAD67 mRNA) techniques, to study the response of DMN GABAergic cells to the degeneration of nigral dopaminergic cells. Our results showed that unilateral dopaminergic cell loss (after injection of 6-hydroxydopamine in the medial forebrain bundle) induces a bilateral and symmetrical increase in both firing rate and GAD67 mRNA levels and a decrease in GAD65 mRNA levels. These findings support the involvement of DMN GABAergic cells in the basal ganglia modifications that follow dopaminergic cell loss, also suggesting its participation in the pathophysiology of Parkinson's disease. The symmetry of effects, together with its recently reported bilateral projections to the thalamus and superior colliculus, suggest that unlike substantia nigra reticulata, DMN is involved in the interhemispheric regulation of basal ganglia, probably keeping their functional symmetry even after asymmetric lesions.

The deep mesencephalic nucleus as an output center of basal ganglia: morphological and electrophysiological similarities with the substantia nigra

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region between the superior colliculus, the substantia nigra compacta, the periaqueductal gray, and the medial geniculate body. Although some data suggest that it is involved in nociception and visceral control, its functions remain unclear. In the present study, by using morphological (combination of anterograde and retrograde tracers with immunocytochemistry and in situ hibrydization) and electrophysiological (firing activity and transynaptic response to striatal stimulation) methods, we show that a subpopulation of DMN cells shares many morphological and electrophysiological characteristics with those of the substantia nigra reticulata (SNR). These similarities include the following: 1) firing rate, firing pattern, and conduction velocity; 2) expression of GAD65, GAD67, and PV; 3) excitatory and inhibitory inputs from the striatum; and 4) projections to the ventral thalamus, superior colliculus, and pedunculopontine tegmental nucleus. Some differences were also found. In comparison with SN, DMN cells and striatal afferents are more sparsely distributed and they show conspicuous contralateral projections to the thalamus and superior colliculus. This suggests that, similarly to the SNR, the DMN acts as an output center of basal ganglia and probably facilitates the inter-hemispheric regulation of these centers.

The connections of the dopaminergic system with the striatum in rats and primates: an analysis with respect to the functional and compartmental organization of the striatum

This Commentary compares the connections of the dopaminergic system with the striatum in rats and primates with respect to two levels of striatal organization: a tripartite functional (motor, associative and limbic) subdivision and a compartmental (patch/striosome-matrix) subdivision. The topography of other basal ganglia projections to the dopaminergic system with respect to their tripartite functional subdivision is also reviewed. This examination indicates that, in rats and primates, the following observations can be made. (1) The limbic striatum reciprocates its dopaminergic input and in addition innervates most of the dopaminergic neurons projecting to the associative and motor striatum, whereas the motor and associative striatum reciprocate only part of their dopaminergic input. Therefore, the connections of the three striatal subregions with the dopaminergic system are asymmetrical, but the direction of asymmetry differs between the limbic versus the motor and associative striatum. (2) The limbic striatum provides the main striatal input to dopamine cell bodies and proximal dendrites, with some contribution from a subset of neurons in the associative and motor striatum (patch neurons in rats; an unspecified group of neurons in primates), while striatal input to the ventrally extending dopamine dendrites arises mainly from a subset of neurons in the associative and motor striatum (matrix neurons in rats; an unspecified group of neurons in primates). (3) Projections from functionally corresponding subdivisions of the striatum, pallidum and subthalamic nucleus to the dopaminergic system overlap, but the specific targets (dopamine cells, dopamine dendrites, GABA cells) of these projections differ. Major differences include the following. (1) In rats, neurons projecting to the motor and associative striatum reside in distinct regions, while in primates they are arranged in interdigitating clusters. (2) In rats, the terminal fields of projections arising from the motor and associative striatum are largely segregated, while in primates they are not. (3) In rats, patch- and matrix-projecting dopamine cells are organized in spatially, morphologically, histochemically and hodologically distinct ventral and dorsal tiers, while in primates there is no (bi)division of the dopaminergic system that results in two areas which have all the characteristics of the two tiers in rats. Based on the anatomical data and known dopamine cell physiology, we forward an hypothesis regarding the influence of the basal ganglia on dopamine cell activity which captures at least part of the complex interplay taking place within the substantia nigra between projections arising from the different basal ganglia nuclei. Finally, we incorporate the striatal connections with the dopaminergic system into an open-interconnected scheme of basal ganglia-thalamocortical circuitry.

The efferent projections of the dorsal and ventral pallidal parts of the pigeon basal ganglia, studied with biotinylated dextran amine

In the present study we have investigated the efferent projections of both the dorsal and the ventral pallidum of the pigeon basal ganglia, using the sensitive anterograde tracer biotinylated dextran amine [Veenman C. L. et al. (1992) J. Neurosci. Meth. 41, 239-254]. Injections of biotinylated dextran amine in the pigeon dorsal pallidum produced numerous fibers and terminals in specific nuclei of the thalamus, hypothalamus, pretectum and midbrain tegmentum. In the thalamus, labeled fibers and terminals were observed in the avian thalamic reticular nucleus, the proposed motor part of the avian ventral tier (ventrointermediate area), the avian parafascicular nucleus (nucleus dorsointermedius posterior), as well as in the avian nucleus subrotundus (which may be comparable to the posterior intralaminar nuclei of mammals). Labeled fibers and terminals were also observed in the avian subthalamic nucleus (anterior nucleus of the ansa lenticularis), in the pretectum (nucleus spiriformis lateralis) and in the avian substantia nigra pars reticulata. Injections of biotinylated dextran amine in the pigeon ventral pallidum produced fibers and terminals in specific centers of the telencephalon, hypothalamus, thalamus, epithalamus, and midbrain and isthmic tegmentum. Labeled fibers and terminals were also observed in the avian subthalamic nucleus and the inmediately adjacent lateral hypothalamus, the avian thalamic reticular nucleus, the avian medidorsal nucleusaand posterior intralaminar nuclei, and the lateral habenula. Finally, labeled fibers and terminals were found in the ventral tegmental area, the avian substantia nigra pars compacta and the midbrain/isthmic tegmentum, which includes the pedunculopontine tegmental nucleus. Our results indicate that both the dorsal and ventral pallida of birds have unique and specific projection patterns, which are very similar to those of their counterparts in mammals. Our study suggests that these avian basal ganglia regions may be related mainly to somatomotor and limbic functions, respectively.

Atlas of the neurons that express mRNA for the long form of the prolactin receptor in the forebrain of the female rat

Prolactin has a variety of important physiological effects on peripheral tissue and on the brain. The behavioral effects of prolactin include the induction of maternal behavior and increased food intake. Prolactin acts via its cognate receptors which have two forms, a short and a long form. The long form of the receptor is predominant in the preoptic area-hypothalamus and is positioned to support maternal behavior since this form is regulated across pregnancy and lactation (Nagano and Kelly [1994] J. Biol. Chem. 269:13337-13345; Sugiyama et al. [1994] J. Endocrinol. 141:325-333). By using in situ hybridization with [33P] labelled cRNA probe specific for the long form of the receptor mRNA(L-PRL mRNA) we have mapped, in brains from 2- and 21-day-old pregnant females, the neuroanatomical distribution of neurons expressing the long form of the receptor. Many neurons with high expression of L-PRL mRNA were located in the anteroventral periventricular nucleus, the medial preoptic area (MPO), specific subdivisions of the paraventricular and supraoptic nuclei, and in the arcuate and ventromedial nuclei. Labelled neurons were also found in limbic system structures such as the bed nucleus of stria terminalis (BST) and the medial nucleus of the amygdala, in a few thalamic nuclei, and in the central gray. All cells throughout the choroid plexus expressed high levels of L-PRL mRNA. The levels of L-PRL mRNA were higher in females on day 21 of pregnancy in the MPO and in the choroid plexus, than in females on day 2 of pregnancy; levels in the ventromedial nucleus of the hypothalamus (VMH) were unchanged across pregnancy. The neuroanatomical distribution of neurons expressing L-PRL mRNA may have special relevance for the mediation of maternal behavior, lactation, sexual behavior, and feeding.

Anterograde axonal tracing with the subunit B of cholera toxin: a highly sensitive immunohistochemical protocol for revealing fine axonal morphology in adult and neonatal brains

We report an improved immunohistochemical protocol for revealing anterograde axonal transport of the subunit B of cholera toxin (CTB) which stains axons and terminals in great detail, so that single axons can be followed over long distances and their arbors reconstructed in their entirety. Our modifications enhance the quality of staining mainly by increasing the penetration of the primary antibody in the tissue. The protocol can be modified to allow combination in alternate sections with tetramethylbenzidine (TMB) histochemical staining of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Using the protocol, we tested the performance of CTB as an anterograde tracer under two experimental paradigms which render other anterograde tracers less sensitive or unreliable: (1) labeling the entire retinofugal projection to the brain after injections into the vitreal chamber of the eye, and (2) labeling developing projections in the cortex and thalamus of early postnatal mammals. Qualitative comparisons were made with other tracers (Phaseolus vulgaris leucoagglutinin, dextran rhodamine, biotinylated dextran, free WGA, or WGA-HRP) that were used to label these same projections. From these observations it is clear that CTB, visualized with our protocol, provides more sensitive anterograde labeling of retinofugal projections as well as of axonal connections in the neonatal forebrain.